1. Field of the Invention
This invention relates to an electric power steering apparatus for applying power from an electric motor to a steering system to lighten the steering force that must be exerted by a driver, and more particularly to an electric power steering apparatus with which when the steering system has been turned to a maximum steering angle position an excessive current is not continuously supplied to the motor.
2. Description of the Related Art
In an electric power steering apparatus for applying a steering assist force (steering assist torque) outputted by a motor to a steering force transmitting system, a current passing through the motor is controlled in correspondence with steering information such as steering wheel steering angle, steering speed and steering force (steering torque) and motion information of vehicle speed, and a steering assist force outputted by the motor is made to act on turning wheels of the vehicle by way of a steering mechanism and causes the turning wheels to turn.
For example, in a rack and pinion type electric power steering apparatus, a steering force from the steering wheel and a steering assist force from the motor are made to act on a rack and move the rack from a neutral position in a left-right axial direction, and this movement of the rack is transmitted by tie rods to turning wheels and causes the turning wheels to turn.
In this kind of electric power steering apparatus, by the rack being made to abut with a gear case or the like at left and right maximum movement positions of the rack (hereinafter called maximum steering angle positions), the rack is prevented from moving beyond the maximum steering angle positions and it is made impossible for the steering wheel to be turned to beyond the maximum steering angle positions. Therefore, when the rack is in a maximum steering angle position it is not necessary for a steering assist force to be supplied from the motor and it is desirable that the steering assist force outputted from the motor be limited to reduce its power consumption.
In Japanese Utility Model Laid-Open Publication No. SHO-60-193868, an electric power steering apparatus is disclosed wherein a microswitch for detecting that the rack has moved to a maximum steering angle position is mounted in a gear case and the steering assist force is reduced on the basis of the output of this microswitch.
However, with a construction having a microswitch for detecting that the rack has moved to a maximum steering angle position, because it is necessary for wiring to be provided between the microswitch mounted in the gear case and a control unit and considerable accuracy is needed in the mounting position of the microswitch, the assembly labor increases.
To overcome the foregoing, in Japanese Patent Laid-Open Publication No. HEI-1-9064, the present inventors have proposed an electric power steering apparatus having steering force detecting means for detecting a steering force of a steering system, steering speed detecting means for detecting a steering speed of the steering system, and overload preventing means for reducing the current flowing through a motor when the steering force detected by the steering force detecting means is above a predetermined value and the steering speed detected by the steering speed detecting means is below a predetermined value.
A block diagram of a control unit of a known electric power steering apparatus having overload preventing means of this kind is shown in FIG. 9 hereof.
Referring to FIG. 9, a control unit 120 has a target current calculating part 121, a steering speed calculating part 122, an overload preventing part 123 and a motor driving part 124. BAT is a battery power source, and the reference numeral 125 denotes a motor current detector for detecting the current flowing through a motor 110 and outputting a motor current signal (hereinafter called the motor current) IM. The reference numeral 126 denotes a motor voltage detector for detecting the voltage impressed on the motor 110 and outputting a motor voltage signal (hereinafter called the motor voltage) EM.
The target current calculating part 121 obtains a basic current that should be supplied to the motor 110 to cause the motor 110 to produce a steering assist torque corresponding to a steering torque on the basis of a steering torque signal Tp outputted from a steering torque detector 112 and corrects this basic current value in correspondence with a vehicle speed signal 114a outputted by a vehicle speed sensor 114. The target current calculating part 121 also corrects the basic current value in correspondence with a steering speed signal (hereinafter called the steering speed) VM outputted from the steering speed calculating part 122 and outputs the corrected current to be supplied to the motor as a target current value (hereinafter called the target current) IT. The target current IT is supplied through the overload preventing part 123 to the motor driving part 124.
The steering speed calculating part 122, which constitutes a steering speed detecting part, estimates a motor speed VM from the motor current IM and the motor voltage EM on the basis of Exp. (1) below and outputs the estimated motor speed VM as a steering speed VM. EQU VM=(EM-IM.multidot.RM)/Kp (1)
where RM is the resistance of the motor and Kp is an induced voltage coefficient.
There is a fixed relationship between the motor speed (the rotational speed of the motor) and the steering speed, and therefore it is possible to treat the motor speed calculated using Exp. (1) as a steering speed VM. The steering speed VM is supplied to the target current calculating part 121 and the overload preventing part 123.
Alternatively, instead of a steering speed VM equivalent to the motor speed VM being estimated by calculation from the current IM and the motor voltage EM, a steering speed detector may be provided on the steering shaft in place of the steering speed calculating part 122 to detect the actual steering speed.
The overload preventing part 123 detects an overload state of the motor 110 and in the overload state reduces the current supplied to the motor 110 through the motor driving part 124 by decreasing the target current IT supplied to the motor driving part 124. For example, when the steering system has been turned to a maximum steering angle position or when a manual steering torque is being detected while the front wheels (turning wheels) are trapped in grooves or the like and cannot be steered, even if a current is supplied to the motor 110 so as to produce a steering assist torque corresponding with this manual steering torque, because the rack shaft cannot be moved any further the motor 110 cannot rotate and consequently power is consumed wastefully and the motor 110 is unnecessarily heated.
To avoid this, in the overload preventing part 123 shown in FIG. 9, when an overload state detecting part 131 detects an overload state it changes a multiplication coefficient K that it supplies to a multiplier 132 to 0.5 or below from a normal value of 1 and thereby causes a corrected target current ITH supplied to the motor driving part 124 to decrease. The overload state detecting part 131 is constructed to determine that the motor is in an overload state and change the multiplication coefficient K from its normal value of 1 to 0.5 or below when the motor current IM is larger than a preset value and the steering speed VM is smaller than a preset steering speed.
The motor driving part 124 has a deviation calculating art 141, a PID control part 142, a PWM signal generating part 143, a gate driving circuit part 144 and a motor driving circuit part 145 consisting of four power field effect transistors connected in an H-type bridge.
The deviation calculating part 141 obtains the deviation between the corrected target current ITH and the motor current IM detected by a motor current detector 125 and outputs a deviation signal 141a. The deviation signal 141a is supplied to the PID control part 142.
The PID control part 142 carries out processing such as proportional, integral and differential processing on the deviation signal 141a and generates and outputs a drive control signal 142a for controlling the current supplied to the motor 110 so that the above-mentioned deviation approaches zero. The drive control signal 142a is supplied to the PWM signal generating part 143.
The PWM signal generating part 143 generates and outputs PWM (Pulse Width Modulation) signals 143a for PWM-operating the motor 110 on the basis of the drive control signal 142a. The PWM signals 143a are supplied to the gate driving circuit part 144. The gate driving circuit part 144 drives the gates of the field effect transistors and thereby drives switching of the field effect transistors on the basis of the PWM signals 143a.
The control unit 120 PWM-controls the power supplied from the battery power source BAT to the motor 110 on the basis of the steering torque Tp detected by the steering torque detector 112 and thereby controls the output power (the steering assist torque) of the motor 110, and when it is detected by the overload state detecting part 131 inside the overload preventing part 123 that the motor 110 is in an overload state the control unit 120 reduces the current supplied to the motor 110 by decreasing the corrected target current ITH supplied to the motor driving part 124.
However, because the overload preventing part 123 of the control unit 120 shown in FIG. 9 is constructed to determine that the motor is in an overload state and reduce the current supplied to the motor when the motor current IM is larger than a preset value and the steering speed VM is smaller than a preset steering speed, when a steering operation is carried out slowly it sometimes happens that although the steering system has not been turned to a maximum steering angle position it is mistakenly detected that the motor 110 is overloaded and the current supplied to the motor 110 is consequently reduced. When the current supplied to the motor 110 is reduced as a result of an erroneous detection like this, because the steering assist torque supplied from the motor 110 is decreased, the steering may become heavy and disconcert the driver.
Therefore, there is a demand for the provision of an electric power steering apparatus with which even when a steering operation is carried out slowly a stable steering assist torque can be supplied from the motor and with which also an excessive current is not continuously supplied to the motor when the steering system has been turned to a maximum steering angle position.